Christchurch police are promising a crackdown on under-age prostitutes and the men who use them amid new claims that increasing numbers of teenagers are selling themselves.

Front-line police officers and street workers say there has been a noticeable rise in the number of young women selling their bodies in recent months.

Although the exact number of under-age prostitutes is not known, police say action is required.

Two years ago, The Press reported young teenagers were selling themselves for drugs, alcohol and food.

But since the Prostitution Reform Act was introduced in 2003, decriminalising the sex industry, The Press has recorded just one case of a person convicted for buying sex from a girl under 18 years.

Inspector Gary Knowles, of the Christchurch police, said officers were worried about young street workers.

"There's no doubt it's happening. Front-line staff are telling us there is an increase in under-age girls working the street and we have gleaned information from street workers that a lot of the girls are under age," he said.

Officers were already policing the city's red-light district to increase street workers' safety, but investigations would be stepped up to get the young girls off the street, including invoking the Children, Young Persons and Their Families Act 1989 to remove them from "a situation detrimental to their health", Knowles said.

"It's not a healthy environment. They are not only subjected to violence, but they're subjecting themselves to social diseases.

"(The act) gives us the power to return them to their guardian or whanau or back into CYF (Child, Youth and Family) custody."

Pimps and clients who knowingly had sex with under-age street workers would also be targeted.

"It's one thing to be over 18 and choose that profession, but we do have an issue with the number of under-age girls we are finding on the street," Knowles said. "I mean, 14, 15 and 16-year-old kids are out there to make money, and they are kids, they look young. You've got to question the morals of the men picking them up. The legislation is designed to protect children put into prostitution. If people are knowingly having sex with minors, they expose themselves to prosecution."

Sex worker Christine (not her real name), 22, said there had "definitely" been an increase of street workers aged 14 to 17.

"Three or four years ago, there were a couple of them, but it wasn't so obvious as it is now because they used to stand in Latimer Square. Now they are full-on down Manchester Street," she said.

The young girls usually stood in groups and had young pimps "on skateboards and stuff", she said. "Usually really old men pick them up. Of course they, (the clients) know they are under-age. Those guys love the really young girls and that's specifically what they want. The younger the better. Even 13 and 14-year-olds - it's more a turn-on."

Most young girls on the street were feeding a drug habit, she said. "I know the young ones are right into P (methamphetamine) and herbals. People drive up and down selling as well, so it's really easy to get."

People who did not believe the numbers of under-age street workers were increasing needed to "open their eyes", she said.

Knowles said decriminalising prostitution had exposed the presence of under-age street workers.